1. Field of the Invention
The present invention relates to an inflatable cuff for blood pressure measurement, which is adapted to be worn on a body portion of a living subject, and particularly to a cuff having a first inflatable bag for pressing the body portion and a second inflatable bag for sensing a pulse wave.
2. Description of Related Art
A cuff, including an inflatable bag, is adapted to be wound around a body portion of a living subject. The bag inflates when a gas or a liquid is supplied into, and consequently the wound cuff presses the body portion.
A cuff may function not only as a device for pressing a body portion, but also as a device for sensing a pulse wave propagating from an arterial vessel. In general, an oscillometric blood pressure measuring device slowly deflates the cuff wound around a body portion to weaken a force by the cuff against the body portion, and determines a blood pressure based on pulse waves continuously received by the cuff during the slow deflation. The device adopts a static pressure of the inflatable bag as a systolic pressure, at a rising point of an envelope of amplitudes of the pulse waves, that is, a point where the amplitude of the pulse waves suddenly raises.
The device may determine an indefinite systolic pressure when the cuff has only one inflatable bag because a fluctuation in pressure in the inflatable bag provides with indefinite rising point of the envelope of the amplitudes of the pulse waves.
To solve the above problem, JP 05-269089 A, U.S. Pat. No. 6,346,083 and U.S. Pat. No. 6,497,668 disclose a cuff having an inflatable bag for pressing a body portion so as to stop a flow of blood in an arterial vessel and an inflatable bag for sensing a pulse wave from the arterial vessel. The cuff disclosed in these references includes an outer bag for pressing the body portion and an inner bag for sensing the pulse waves. A device using this cuff provides a relatively definite systolic pressure with a relatively definite rising point of the envelope of the amplitudes of the pulse waves sensed by the inner bag, because the inner bag is substantially located in the center of and on a body portion side of the outer bag and no pulsation directly propagates to the inner bag when the arterial vessel pulsates again around an upstream end of the outer bag.
An uneven surface of the body portion, such as an ankle, is difficult to fittingly wind with the cuff. The cuff having the outer bag for pressing the body portion and the inner bag for sensing the pulse waves disclosed in the above references cannot provide with a definite systolic pressure because of an indefinite shape of the envelope of the amplitudes of the pulse waves sensed by the inner bag if the inner bag is not fully wound on the uneven surface of the body portion.